Tracking systems (also known as navigation systems) assist surgeons during surgeries that require the precise locating of instruments. Such surgeries include neurosurgery and orthopedic surgery. The tracking system tracks the position and orientation of an instrument during the procedure and often displays the position and/or orientation of the instrument on a monitor in conjunction with a preoperative image or an intraoperative image of the patient (preoperative images are typically prepared by MRI or CT scans, while intraoperative images may be prepared using a fluoroscope, low level x-ray or any similar device). Alternatively, some systems are image-less in which the patient's anatomy is instead registered and mathematically fitted with an anatomical model.
Prior art tracking systems typically employ a camera that detects a tracking device located on the instrument. The tracking device has a plurality of optical markers such as light emitting diodes (LEDs) to determine the position and orientation of the instrument. The position of the instrument usually correlates to the coordinates of a working end of the instrument in three-dimensional space, the x, y, z or Cartesian coordinates, relative to the camera. The orientation of the instrument means the pitch, roll, and yaw of the instrument. When both the position and the orientation of the instrument are defined, the relative position of that instrument is known to the tracking system.
Orthopedic surgeons have been using tracking systems for some time to assist in properly locating and positioning cutting jigs. Cutting jigs are used to resect bone for the purpose of preparing joints to accept replacement implants. The time required to position and secure a cutting jig can appreciably add to the overall time required to perform a joint replacement surgical procedure. It should be appreciated that the cutting jig must be accurately positioned Imprecise positioning of a cutting jig can contribute to a less than ideal surgical outcome. As a result, there has been a movement to eliminate the use of cutting jigs. Instead, surgeons would rely solely on tracking the instrument to ensure that the cutting portion of the instrument does not stray beyond a predefined boundary.
In such tracking systems both the instrument and the material being cut are outfitted with trackers such that the tracking system can track both the position and orientation of the instrument and the material being cut such as a bone. The instrument is held by a robot or other articulation mechanism that provides some form of mechanical constraint to movement. This constraint limits the movement of the instrument to within a predefined boundary. If the instrument strays beyond the predefined boundary, a control is sent to the instrument to stop cutting. Such systems are shown in U.S. Pat. No. 5,408,409 to Glassman et al.
It has also been proposed in the prior art that the instrument be used free hand without the aid of cutting jig, guide arm or other constraining mechanism to establish the location to which the cutting implement at the end of the instrument is applied. See, for example, U.S. Pat. No. 6,757,582 to Brisson et al.